1. Field of the Invention
The present invention relates to a pixel clock phase locked loop for a laser scanner and, more particularly, to a pixel clock phase locked loop which compensates for low frequency motor hunting, polygon motor cogging and facet jitter.
2. Description of the Related Art
A laser scanner such as a raster output scanner is used to transmit image information to an imaging surface such as a recording medium. A light source, such as a laser light source, generates a light beam which is modulated in accordance with image information contained in video image signals or pixels. The modulated light beam is applied to a rotating, multi-faceted polygon which scans the modulated light beam across the image plane of the imaging surface. Each facet of the polygon is mirrored. The polygon is spun by a motor, the motor speed controlling image resolution in the direction of movement of the imaging surface (i.e., the Y-direction). Image resolution within a scan line (i.e., the X-direction) is a function of the image signal or pixel rate. The resolution in the direction of scan is determined by the image signal or pixel clock frequency. Each mirrored facet of the polygon provides image information corresponding to one horizontal scan line.
Motor speed errors often occur at different frequencies which cause image distortion in the scan line direction. These errors include motor hunting in which the speed of the motor which spins the polygon changes slightly at a low frequency, e.g., less than 10 Hertz, motor cogging which is a motor aberration occurring at a motor cogging frequency equal to the number of motor poles multiplied by the number of revolutions per second, and facet jitter which is a motor aberration occurring at every scan line, i.e., every facet. Particularly when color printing is being performed and pixels must be registered for each of a plurality of separations, it is critical for pixel registration to be accurate to prevent distorted, blurred images from resulting. Errors also occur in individual polygon facets such as differences in radius and angularity of each facet of the polygon. These errors also distort the resulting image.
Different techniques have been developed for correcting such errors. One of these techniques includes the use of a pixel clock phase locked loop. Such phase locked loops have, for example, been used to generate pixel clock pulses at a frequency which is a function of polygon velocity, the phase locked loop monitoring variations in the polygon velocity and adjusting the clock frequency to maintain image size constant in the direction of the scan line. These phase locked loops are typically designed to compensate for the relatively low frequency motor hunting. Known phase locked loops correct the fast scan line based on previous scan line error. Facet jitter, however, is unique to each facet. Accordingly, previously scan line error is the wrong error information to be used for correction in the next scan line. A pixel phase locked loop which corrects for varying frequencies of error is thus desirable.
U.S. Pat. No. 4,204,233, to Swager, assigned to Xerox Corporation, discloses a system for correcting a facet error which changes the rate of a bit clock based on errors of individual facets of a rotating polygon. At the time of a start-of-scan signal, a bit clock counter is initiated. The error for a particular facet is determined by the interval between a scan line bit count output and an end-of-scan output. The facet error is represented by a binary number corresponding to the interval. The error for a particular facet is stored in a memory location corresponding to that facet. When the facet is utilized, the error previously stored in the memory location for that facet is used to control an oscillator so that the output frequency corresponds to the frequency required to compensate for velocity errors caused by that facet. After the facet is scanned, the error signal previously stored in memory for that facet is updated. During the time between the end-of-scan signal and the start-of-scan signal for a scan line, the error for the next facet is read out of memory. Because the pixel clock is itself used to measure, or count, the error, the reference allows only an accuracy to within plus or minus one pixel clock per scan line. Thus, if used in conjunction with a typical pixel clock which runs up to a maximum of 100 MHz, the accuracy to which error correction can be achieved equates to only within plus or minus 10 nanoseconds correction per scan line.
U.S. Pat. No. 4,270,131 to Tompkins et al discloses a system for electronically correcting for errors in a laser scanner. Corrections are provided for each facet of a spinner mirror and are stored in random access memories until the same mirror again reflects a laser beam across a photosensitive drum. After several spinner revolutions, the random access memory is stabilized. The start-of-scan signal is used to locate all of the facets at the beginning of each scan line along a vertical line. There is no correction for errors attributed to different frequencies of motor speed.
U.S. Pat. No. 4,257,053 to Gilbreath discloses a laser graphics plotter for plotting data on a recording medium as selectively positioned pixels of variable intensity. A spot placement means is provided to generate a spot placement signal which cooperates with plot data to modulate the laser beam. The spot placement means provides error correction for facet-to-facet and facet-to-axis errors in the rotating mirror to permit high resolution plotting on wide format film. The device compensates only for manufacturing tolerances.
U.S. Pat. No. 4,349,847 to Traino, assigned to Xerox Corporation, discloses a raster output scanner having a movable imaging member and imaging beam for exposing the imaging member to create images thereon. A rotating polygon scans the beam across the imaging member in line-by-line fashion while the beam is modulated in accordance with pixels input thereto. A clock provides clock pulses for clocking the image pixels to the modulator. The polygon velocity is controlled to maintain a predetermined velocity relationship between the imaging member and polygon. Accordingly, the device compensates only for velocity variations.
U.S. Pat. No. 3,867,571 to Starkweather et al, assigned to Xerox Corporation, discloses a flying spot scanning system which utilizes reflected light from a multi-faceted rotating polygon. In each scanning cycle, information is transmitted to a scanned medium by modulating light from a light source in accordance with a video signal. To assure a uniform spot size at the scanned medium, an optical convolution of elements is selected in combination with the light source. The rotation of the polygon is synchronized in phased relation to the scan rate used to obtain the video signal. The device provides no compensation for motor speed error at different frequencies.
While the related art attempts to compensate for various errors affecting pixel registration, the art does not compensate for errors occurring at various frequencies of motor speed such as motor hunting, polygon motor cogging and facet jitter.